1. Technical Field
The present inventions each relate to methods and arrangements for manufacturing spray formed metallic articles; more specifically the inventions relate to such inventive aspects as heat treatment processes for minimizing internal stresses and deflections in produced articles, manipulating temperature and the time periods for hold certain temperatures to establish prescribed multi-phase metallic compositions in produced articles also for minimizing internal stresses and deflections in produced articles, and a unique one dimensional based model utilized for affecting feed-forward control over the spray form process.
2. Background Art
It is a known process to spray form certain articles using moltenizing arc guns having metal feed wire supplied thereto. Further, it is known that volumetric changes occur during cooling of the metal that can produce significant detrimental effects in the finished product, one of the more significant of which is typically manifest as internal stress that is trapped within the substantially rigid article after its manufacture. It is not uncommon for stresses of magnitudes high enough to warp or otherwise cause deformation and deflection in the finished article to occur in uncontrolled spray processes, and even minor deflections due to internalized stress can render conventional spray form processes unusable when precision tooling is required for particular finished products or articles. In another aspect, as the technology and processes for spray forming metallic articles advance, the manufacture of progressively larger monolithic bodies becomes feasible. As a result, however, the volumetric changes experienced during the cooling of the metal in such larger spray formed bodies is becoming more pronounced due, for example, to their greater sizes and thicknesses. The detrimental effects of these volumetric changes experienced within a spray formed article have long been appreciated; not the least of which can be, and often is, the inducement of internal stresses within the article itself.
It is known that molten steel can undergo various phase changes, for example, from austenite to ferrite, pearlite, bainite, martensite, and various combinations thereof as it cools, and that these phase changes involve positive volumetric changes. Previously, it has been postulated that the transformation to martensite can offset the stresses caused by shrinkage that also occurs during cooling. The focus of this idea was that a balance between the positive volumetric changes and the thermal contractions could be effected by the transformation to martensite.
When steel is initially sprayed and still at a high temperature, it is typically one-hundred percent austenite, and as the steel begins to cool, the austenite begins to change into other sister phases. At a relatively high temperature, the first phase transformations are primarily into ferrite and pearlite. As the temperature moves lower, the next transform is into bainite, and at the lowest temperature, transformation to martensite occurs. Even though it was known that these transformations were occurring as the steel cooled, it has been the martensite transformation which has been primarily capitalized upon to provide stress relief to the spray formed body or article.
A current approach to controlling the spray forming process has been through temperature control. In such an approach, temperature is used as an input for robotically manipulating the spray guns. In this approach, the moltenized metal spray is produced using, for example, a number of twin-wire arc plasma torches or guns. The movement and performance of the guns may be automated via computer/robot controls, the surface temperature(s) of the article may be monitored, and the spray pattern responsively adjusted to control the temperature of the body being sprayed. This exclusively temperature based control process, however, is only suitable when considering transformations from austenite to martensite which is only a function of temperature. It is not suited to transformations of austenite to ferrite, pearlite, or bainite because these phase changes are only partly temperature based. Because these transforms are diffusional processes that are also time-based, as well as temperature based, such transformations can occur even when temperature is held constant. Therefore merely monitoring and controlling the article""s surface temperature fails to fully address the problem of internal stresses that occur during the spray forming process.
During the spray form process, the temperature of the moltenized metal droplets that are sprayed onto the ceramic model are significantly elevated above the temperature of the ceramic model and the surrounding atmosphere. Once the droplets leave the spray gun and land on the ceramic model, they become a constituent component of the article being spray formed. A portion of the heat energy input to moltenize the feed metal wire travels conductively into the ceramic model after landing, while a portion of the imposed heat remains in the body of the article being spray formed. The balance of the heat energy is dissipated out into the surrounding atmosphere which is typically the interior space of the spray form cell or enclosure in which the spraying process is taking place. As a result, different parts of the microstructure have traditionally been permitted to have different temperatures during and after the spraying process. This is especially true, for example, in the case of a large stamping tool, such as that required for stamping an automobile inner hood, if the tool were sprayed as a unibody monolith.
In another aspect, spray formed articles having complex shapes that cause different regions of the article to have relatively different locally exposed surface areas tend to cool at different rates amongst these several regions. This characteristic, in turn, affects the kinetics of the body""s overall cooling profile. Different areas of an irregularly shaped article, especially an article having many undulations, tend to cool at different rates, for example, because the presence of the undulations tends to restrict heat transfer. Thus, areas within depressions of the undulations tend to be hotter than areas that protrude with a proportionately greater exposed surface area. As a result, one part of the article being sprayed can be in the bainite transformation phase, while another part is in the martensite start region.
Further, when spraying is discontinued and the sprayed article is allowed to cool to room or ambient temperature, different temperatures will begin to occur across the sprayed body. As a result, those areas loosing temperature more quickly begin to traverse the phase transformations sooner than those areas that are more heat retentive. This phenomenon is even more pronounced with a sprayed article that has a complex shape, such as those including undulations or apertures, which causes certain areas to be warmer than others until the final cooling temperature is reached and the article assumes a uniform temperature, such as equal to the temperature of the spray form cell""s interior. When such articles are simply allowed to cool to room temperature in an uncontrolled manner, significant distortions are likely to occur in the article because of discontinuities across the phase transformations and stresses are created in the bodies because of these different cooling rates.
Currently available technology provides the user with an ability to monitor the exposed surface temperature of an article being spray formed. However, in spite of the recognized need, a continuing failure in the art has been a lack of means and method to accurately predict, monitor and control the more elusive, but more comprehensive, time and temperature dependent phase constituencies and volumetric changes that occur during the spray forming process. Consequently there has been a continuing inability to affect proper control over the time and temperature based phase constituencies and volumetric changes during the spray forming process for obviating the problems associated with internal stresses induced in the article being spray formed.
In view of the above described deficiencies associated with currently available spray form processes when considering time and temperature dependent phase and volumetric changes within the article being formed, the present inventions have been developed to alleviate these drawbacks and to provide further benefits to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to illustrative embodiments of the inventions.
FIG. 1 shows an example of a basic graph plotting time, temperature, and phase transformation for a typical carbon steel, also known as a TTT curve, which indicates several different general zones. In a top portion of the graph at temperatures, for example, above about 750 C. sprayed metal remains in a stable austenite phase regardless of the time held at this temperature. Moving down on the temperature scale (y-axis), at the left side of the mid-portion of the graph, an austenite phase is also found, but in an unstable condition. This condition is based on lower temperatures between, for example, about 210 and 720 C. for the particular steel for which the TTT curve is plotted, at which the metal is sprayed on the ceramic model.
Moving to the right on the graph, it can be seen that this unstable austenite phase lasts but a short period of time, which is apparent based on the x-axis that shows time on a logarithmic scale. Moving to the right as time passes, the unstable austenite zone is left behind and a middle and transitional zone is entered which is characterized by some or all of the austenite converting to a ferrite and/or pearlite phase of metal. As more time passes, a third zone is encountered which is characterized by conversion of austenite to bainite. A fifth zone is located at the bottom portion of the graph and is characterized as a martensite zone. Moving through any of the zones, the conversion of austenite to the indicated phase (ferrite or pearlite, bainite, or martensite) is gradual. Therefore, depending on time, it is possible to move across a multitude of zones, resulting in a multitude of different material phases in the finished spray formed article.
Each of the transformations includes a certain degree of inherent volumetric expansion of the constituent metals. In the past, attempts have been made to capitalize on this expansion (potentially causing compressive strains in the spray formed article) to counteract contraction of the metal resulting from cooling, which would otherwise cause tensile strain to be induced in the spray formed article. An aspect of the present invention(s) includes an enhancement to these concepts and an appreciation and control of certain phenomenon which enable the inventions. Such enhancement involves, for example, an appreciation that the two variables of the graph of FIG. 1, the same being temperature and duration maintained at particular temperatures, can be manipulated to achieve more precise volumetric expansion in the spray formed article. That is to say, by manipulation of temperature, and the periods of time that certain temperatures are held, it is possible to xe2x80x9cmove around,xe2x80x9d and into and out of the various phase transformation regions.
It should be appreciated that once a portion of the original austenite phase has been converted, it cannot generally convert to yet another metallic phase. The exception, of course, being that a reconversion back to austenite can be accomplished should the temperature be elevated quite high, such as above about 720 C. as represented in FIG. 1. This-situation, however, is not treated in the present disclosure. That being said, when considering feasible temperatures for spray form processes, it is only possible to achieve a one-hundred percent conversion of austenite to martensite if the application temperature either begins below about 210 degrees C., or quickly drops below that temperature before crossing the interface line into the pearlite-ferrite (middle) zone. Once a certain amount of time has passed causing a portion of the austenite to convert to pearlite, ferrite, or bainite, that converted portion cannot convert to martensite, even if the temperature is sufficiently lowered into the martensite zone. It must be appreciated that it is still likely that a portion of austenite phased metal still remains in the multi-phase xe2x80x9cmixturexe2x80x9d which constitutes the sprayed tool or article. This austenite portion of the metal that has not converted because sufficient time was not spent in the intermediate pearlite-ferrite zone, can be converted to bainite if the temperature is held steady or raised. If the temperature is lowered so that entry is made into the martensite zone, whatever portion of the austenite that remains unconverted at that time is available for conversion to martensite, which is substantially exclusively temperature dependent.
One aspect of the presently disclosed inventions involves the controlled manipulation of both temperature and time for strategic phase changes that result in a specific and planned volumetric increase. This manipulation is made based on ongoing spray parameters, such as the heat energy added to the wire being moltenized and sprayed to form the article. Another aspect of the present invention considers adding heat and raising the temperature after the article""s temperature has dipped down into the martensite zone, taking the temperature back up into any of the three mid-zones (austenite, pearlite-ferrite, and bainite) before complete conversion to martensite occurs. Referring to FIG. 1, it must be remembered that horizontal progression across the three mid-zones is time dependent. In other words, the temperature must be maintained within the mid-zone range for the indicated requisite period(s) of time for one-hundred percent conversion of the austenite to be affected. Otherwise, there will remain a mixture of mixed-phase metals, with that portion which remains as unconverted austenite still being available for conversion to ferrite, pearlite, bainite, and/or martensite depending upon subsequent temperatures levels and, and the durations for which those temperature levels are held. It should be appreciated that the prescribed temperature manipulation may be affected during the spraying process, or after the article has been completely formed.
In one aspect of the inventions, it has been appreciated that one of the reasons that stress can be minimized in a spray formed article is that through purposeful control over magnitude and duration of imposed temperatures, the body of the article can be formed to be of mixed and interspersed metal phase makeup. That is to say, after the spray form process is completed and the article cooled and ultimately removed from the ceramic model upon which it has been sprayed, the constituent metal phase makeup can be controlled to be a mixture of martensite, pearlite-ferrite, and/or bainite. During the spray process, or because of post-heat treatment of the body after termination of the spray process, certain portions of austenite phased metal may also be retained until the temperature is lowered causing martensitic transformation, or sufficient time passes permitting the austenite to convert to pearlite-ferrite or bainite.
Certain of these interspersed phases are xe2x80x9csofterxe2x80x9d and/or more malleable than the other surrounding phases. As a result, these more malleable constituent phases act as buffers and absorb the expansive affects of a phase transition which has occurred nearby. That is, the harder expanding phases can press into the more malleable phases which tend to xe2x80x9csquishxe2x80x9d out of the way. Additionally, the harder and less yielding phases are able to xe2x80x9cslidexe2x80x9d across the more malleable portions. The deformation or xe2x80x9cgivexe2x80x9d of the softer phased material is of a plastic nature, as opposed to elastic nature, and therefore there is no tendency for recoil or tension back to the pre-deformation configuration. As a result, strategic formation of intermixed metal phases has been discovered to avoid and minimize the inducement of stress and strain in the finished article. For this affect to be experienced across the article, the commingling of these different phases must be induced in the body of the spray formed article. The effects of this type of manipulation have heretofore gone unrecognized and therefore have not been capitalized upon via purposeful control of the spray form process. It should be appreciated that this control may be exercised during and/or after the actual spraying processes are complete.
A further aspect of the presently disclosed inventions involves manipulating the temperature of the article being spray formed in such ways as to bring all areas of the sprayed article to a uniform temperature to enable cooling at a more nearly uniform rate across the article and to enable the avoidance, for example, of different proportional combinations of metal phases across the article which could result in imbalanced stress relief. In spray forming processes, it is known that complex shapes and large articles present a problem when a uniform temperature is desired to be maintained across the entire body being spray formed, particularly when the entire spraying process is considered. Based on the descriptions above, the need to be able to control temperature changes in the body of the article being sprayed is easily appreciated. Therefore, the present aspect of the invention contemplates the utilization of heat treatment processes that assure that the metallic organization, prior to cooling, has proper phasing so that when cooled, the thermal shrinkage factor for the article is counteracted. Therefore, the entirety of the sprayed body may be held above a certain threshold temperature thereby enabling controlled conversion between austenite to martensite, as well as other phase transformation throughout the body. This control technique is a key to being able to xe2x80x9cscale upxe2x80x9d traditional processes for utilization in forming progressively larger spray formed bodies; exemplarily, on the order of eight feet by eight feet. Tools of this size may be used to stamp-manufacture such large items as automotive hood and trunk or boot covers. Previously, such large tools could not be manufactured as a unibody or monolith using spray form techniques because unacceptable warping of the finished product or tool could not be avoided.
Implementation of a pre-heat treatment aspect of the present invention before the spray forming process begins involves preheating one or more of the interior space of the spray forming cell, related enclosure(s), and/or the mold substrate upon which the moltenized metal is sprayed. In a process performed according to the teachings of the present invention, application of the metallic spray forming material onto the mold substrate is initiated inside the heated cell. Because the heated environment of the cell can be held nearly constant, or varied as desired by the operator, substantially uniform combinations of metallic phases can be caused for inducing near uniform phase transformations and resulting mixtures of commingled phases across the spray formed body. For example, controlled transformations from the austenite phase can be fostered based on temperature manipulations within the cell. Control of the temperature variations is guided by a predetermined relationship between the initial application temperature of the spray forming material and its correlation to initial temperatures of either or both of the preheated cell environment and the preheated mold substrate.
Implementation of heat treatment during the spray forming process involves applying the metallic spray forming material onto the mold substrate under heated environmental conditions which can be manipulated to cause substantially homogenous metallic phase transformations from the austenite phase, for example, via manipulation of either or both of the substrate temperature and the spray forming cell""s environmental temperature. Sometimes substantially homogenous metallic phase transformations are not desired, but instead customized characteristics are required. In these cases, the temperature of the spray form environment can be appropriately controlled to cause the desired effects through varied metallic phasing across the sprayed metal article.
Implementation of post-heat treatment after spray forming has ended, according to that aspect of the present invention, can include further heating, but more typically involves controlled cooling of the cell environment. The controlled temperature drop may be uniform, or quite abrupt at certain strategic times. For instance, certain transformations are time based, as well as temperature based. This can be appreciated when considering FIG. 1. Therefore, the controlled descent from the heated temperature can be used to cause substantially homogenous, or controlled mixtures of the metallic phase transformations and final phases throughout the resulting spray formed metallic article or tool. Desirably, this causes a substantially homogenous distribution of commingled metallic phases consisting of predetermined proportions of at least bainite phases and martensite phases. By purposefully imposing such a commingled distribution throughout the spray formed body, stress has been found to be more effectively dissipated by the cooling body. Among other reasons, this stress dissipation is accomplished by the inducement of interstitial or mixed phases in which at least one is more susceptible to plastic deformation at lower shear levels than the other(s). As described above, this characteristic facilitates relative xe2x80x9cslidingxe2x80x9d in the softer phases by the less yielding phases which are also typically volumetrically more expansive upon cooling. This combination of characteristics contribute to the present inventions"" successful counterbalance of shrinkage resulting from the cooling of the article which had heretofore caused internal stress, and even worse, warping of the finished article.
Another aspect of the present inventions makes use of what is referred to as one-dimensional modeling to control the spray forming process. In this model, characteristics of a geometrical point are quantified by iterative detection, such as repeatedly taking surface temperature readings using a pyrometer as more and more metal is sprayed. At specific times during the spray forming process, key properties of the body being sprayed are measured and provided as input to the model as initial conditions; the model then uses an optimization algorithm to determine the best control scheme to use until the properties are next measured. In this way, this system of one-dimensional modeling may be characterized as being of the feed-forward type.
Temperature is but one example of the type of data that might be collected at each sampling time to be used as input into the model. Exemplarily, surface temperatures of the spray formed article may be iteratively sensed using a pyrometer. The one-dimensional model, using both historical data and presently sensed data, quickly determines how the spray forming system should be controlled and operated during the next time lapse until the input data is read again. Conceptually, it can be considered that certain characteristics of a core or column representing xe2x80x9ca pointxe2x80x9d down through the depth of the article is sensed in layers. The lower layers are thenceforth modeled or theoretically represented after their actual scan since those layers are now contained below the surface of the article and not susceptible to having most qualities directly measured again. FIG. 1A provides an illustration of such a modeled column representative of actual characteristics. The more xe2x80x9ccolumnsxe2x80x9d that are detected and analyzed, such as a honeycomb configuration of columns, the greater the proportion of the whole of the body of the article being spray formed that can be modeled.
An alternative version of the one-dimensional modeling process described above may be based solely on an input value, rather than a sensed measurement. If that is the case, an original input may be provided to such a model for initiating control of the entire balance of the process. Still further, such a model may be utilized to xe2x80x9cvirtuallyxe2x80x9d analyze different spray conditions without incurring the time expenditure and cost of laboratory testing.
An important advantage of the one dimensional modeling approach is that it can be performed (calculated) very quickly using algorithm-defined, computer-based modeling strategies. In other words, each point-column can be quickly computed and information deduced about characteristics of the particular column at different depths within the spray formed article. The more frequently the sensed points and corresponding columns are spaced across the article, the more continuous the information that can be deduced. Consequently, the more points that are reported, the greater the proportion of the sprayed article that can be analyzed by the one-dimensional model, and the better the predictions will be about how best to modify the spray gun control parameters for the next layers. By straightforward extension of this principle, the one-dimensional modeling program can be written to consider regional characteristics based on a collection of adjacent columns.
The beneficial effects described above apply generally to the exemplary devices, mechanisms and method steps disclosed herein with regard to real-time and predictive monitoring and control of metal spray form techniques. The specific structures and steps through which these benefits are delivered will be described in greater detail hereinbelow.